Delta modulators, also called sigma delta or delta sigma converters, are closed loop systems with quantized feedback that is synchronized to a clock signal. The delta modulator is one form of a charge balance loop. The basic elements of a delta modulator include an amplifier connected as an integrator, a comparator connected to the output of the integrator and a quantizing element connected in a feedback loop between the output of the comparator and an input to the integrator. The quantizing element is typically a D-type flip flop. An analog input signal and a digital error signal derived from the flip flop output are provided to the input of the integrator. The comparator determines whether the integrator output is above or below a prescribed reference level. The flip flop output is a data stream synchronized to the clock signal and includes information representative of the amplitude of the analog input signal.
The data stream from a delta modulator loop is typically supplied to a digital filter which extracts from the data stream information regarding the input signal amplitude. In a conventional delta modulator, the duty cycle of the data stream averaged over a prescribed time period is a measure of the input amplitude. The configuration wherein a delta modulator is followed by a digital filter can be utilized as an analog-to-digital converter. One factor limiting widespread use of such analog-to-digital converters in the past has been the complexity of the digital filter circuits required to process the data stream. Large scale integrated circuits have made the implementation of this type of analog-to-digital converter more feasible.
Several factors limit the accuracy of the delta modulator. One limiting factor is the open loop gain of the integrator. The error is inversely proportional to the open loop gain. For example, if the open loop gain is 1,000, the accuracy is no better than one part in 1,000. Therefore, efforts have been made in the prior art to provide integrators with very high open loop gains. Nonetheless, it is very difficult to obtain accuracies in excess of 16 bits. For 20-bit accuracy, an open loop gain on the order of 10.sup.8 is required. Integrators having such high open loop gains are extremely unstable and are very susceptible to oscillation.
It is a general object of the present invention to provide improved charge balance loops.
It is another object of the present invention to provide improved delta modulators having high accuracy.
It is a further object of the present invention to provide delta modulators wherein accuracy is not limited by the open loop gain of the integrator.
It is a further object of the present invention to provide delta modulators wherein positive feedback is utilized in the integrator to improve accuracy.